Looking at the descr_character_trait file only answer I got is that there's threshold of 4 so it's quite hard to say what the actual amount of units would it be, descr file also seems to point out that best chance for a character to get good recruiter traits is to build a city sized barracks (lvl 3 MIC).

In my romani campaign those fm's who had recruited about 16 units had always received atleast army connections and they didn't even build MIC's so I am pretty sure that expert recruiter will pop out for your faction leader if he recruits atleast 30 units (considering the fact that sharp/charismatic are likely to receive those easier).